Preparation of magnetic activated carbon fibers@Fe3O4 by electrostatic self-assembly method and adsorption properties for methylene blue

Nano-Fe3O4 was loaded onto coconut-based activated carbon fibres (CACF) using an electrostatic self-assembly method. The effects of the mass ratio of CACF to nano-Fe3O4, loading time, pH and temperature on the loading effect were investigated and ideal loading conditions were determined. To study the adsorption performance of MACF@Fe3O4 for methylene blue, the effects of the initial concentration, pH and time on the adsorption were investigated and the working conditions of adsorption were established. MACF@Fe3O4 was systematically characterized. Adsorption kinetics were investigated under ideal conditions. The ideal loading conditions for MACF@Fe3O4 were as follows: mass ratio of 1:1, 20 min, pH 9.36, 22.5°C. The saturation magnetization of MACF@Fe3O4 was 48.2263 emu·g−1, which could be quickly separated under an external magnetic field. When the dosage was 0.010 g, the adsorption rate reached 97.29% and the maximum adsorption capacity was 12.1616 mg·g−1. The adsorption process conformed to pseudo-first-order kinetics during the first 15 min and pseudo-second-order kinetics during 20–120 min. The equations were ln(Qe-Qt)=2.2394-0.0689t and tQt=0.0774 + 0.5295t, respectively. The isothermal adsorption model showed that MACF@Fe3O4 was more in line with the Langmuir model, indicating that the adsorption process was mainly monolayer adsorption. The thermodynamic analysis results showed that the adsorption process of MB by MACF@Fe3O4 was an endothermic process. In this study, MACF@Fe3O4 with high adsorption capacity and easy separation from coconut palm fibres has good application prospects in the field of adsorption, which can promote the high-value utilization of coconut palms.


Introduction
With the continuous increase of coconut production in southern China, the total amount of coconut palm fibre by-products is also increasing.The coconut palms are natural filamentous fibres extracted from coconut shells.They are moisture-proof, antibacterial, harmless and recyclable.At present, the degree of utilization of coconut palm fibres is low, and there is an urgent need to develop high-value materials, such as bio-activated carbon fiber (BACF).Coconut palm fibre is rich in lignin, hemicellulose and cellulose, which is a good raw material for the preparation of activated carbon (AC).AC is considered to be an ideal adsorbent in wastewater treatment systems.Especially the problem of printing and dyeing wastewater treatment, printing and dyeing wastewater contains methylene blue (MB), malachite green, indigo carmine and so on [1][2][3][4].However, it is difficult to separate and recover AC after use [5], especially powdered AC, which must be separated from water by filtration, coagulation, flocculation, clarification and sedimentation [6], resulting in increased costs.This industrial problem is urgently solved.Recently, by loading ferrum (Fe), Cobalt (Co), Nickel (Ni) and other magnetic metals or their oxides to modify AC [7][8][9][10][11][12], the separation of adsorbent can be achieved under the action of an external magnetic field.However, applications of Co and Ni in medicine are limited.In contrast, ferrosoferric oxide (Fe 3 O 4 ) is an iron compound that can be magnetized and is the most commonly used non-toxic magnetic material.The reported carbon (C)-based adsorbents loaded with Fe 3 O 4 include magnetic graphene [13], magnetic biochar [14], magnetic covalent organic frameworks [15], magnetic metal organic frameworks [16], etc.However, there are few reports on the study of coconut-based activated carbon fibres (CACF) [17] loaded with nano-Fe 3 O 4 .The preparation of ferromagnetic AC composites with coconut palm fibre as biomass raw material is an important way to promote the comprehensive utilization of coconut processing by-products and increase the added value of products.
Therefore, in this article, magnetic activated carbon fibre @ Fe 3 O 4 (MACF@Fe 3 O 4 ) was prepared by electrostatic self-assembly method using CACF as raw material and systematically characterized.The ideal preparation conditions of MACF@Fe 3 O 4 and the adsorption conditions of MB were determined.The adsorption kinetics, adsorption isotherms and adsorption thermodynamics were studied.

Material and reagents
Coconut palm fibres were provided by Shanghai Zhao-Kuo Purification Equipment Co., Ltd., and nano-Fe 3 O 4 was provided by Shanghai Aladdin Biochemical Technology Co., Ltd.The remaining conventional reagents were analytical reagents.

Preparation of MACF@Fe 3 O 4 2.2.1. Preparation of CACF
Based on the authors' previous research [17], a sufficient amount of coconut palm fibres was placed in a crucible and sent to a high-temperature box-type resistance furnace for carbonization at 600°C for 120 min.After cooling to room temperature, samples were removed and ground into powder.Carbonized coconut palm fibres (10.0 g) were weighed and mixed with KOH at a ratio of 1:2 and 75% ethanol was added.The mixture was heated and stirred until the liquid was completely volatilized.The carbonized coconut palm fibres that fully absorbed KOH were placed in a crucible, in a high-temperature vacuum tube furnace and activated at 900°C for 150 min under nitrogen.The crude product was rinsed several times with distilled water, and CACF was obtained after drying.

Single factor experiment
Referring to the electrostatic self-assembly method of Wei et al. [18] 0.010 g of CACF was weighed according to different mass ratios, and nano-Fe 3 O 4 was added to 50 ml of distilled water.The pH of the mixture was adjusted to a fixed value and the mixture was allowed to stand at a certain temperature for a certain period.The effects of mass ratio, preparation time, pH and preparation temperature on the adsorption effect were investigated.After filtration, the residue was dried at 70°C for 120 min to obtain MACF@Fe 3 O 4 .
The 0.010 g MACF@Fe 3 O 4 was put to 50 ml of 4.0 mg•l −1 MB and shaken at 25°C for 60 min under natural pH (natural pH refers to the state when pH was not adjusted).The change in the dye concentration was monitored using a UVmini-1280 ultraviolet-visible spectrophotometer (Shimadzu Corporation).

Evaluation of magnetic separation effect
The 0.020 g MACF@Fe 3 O 4 was added to a sample bottle, 20 ml of distilled water was added and the mixture was shaken to simulate the adsorption.A circular magnet with an outer diameter of 80 mm was placed beside the sample bottle, and the magnetic separation of MACF@Fe 3 O 4 was observed after approximately 1 min.

Measuring method
A series of aqueous solutions of MB at different concentrations were prepared, and the concentration-absorbance curve at 664 nm was determined.The standard curve equation was A = 0.2305C − 0.0030 (R 2 = 0.9989).The adsorption capacity (Q e ) was calculated using equation (2.1) and the adsorption rate (D) was calculated using equation (2.2): where C e (mg•l −1 ) is the residual concentration of MB, C 0 (mg•l −1 ) is the initial concentration of MB, V (l) is the volume of the MB solution and m (g) is the mass of the MACF@Fe 3 O 4 .

Structural characterization
The external surfaces of the CACF and MACF@Fe 3 O 4 were observed and analysed using a Regu-lus8220 SEM (Hitachi Limited), and the elemental composition was analysed using EDS.A Vertex 70 FTIR was used to analyse the chemical groups of CACF, nano-Fe 3 O 4 and MACF@Fe 3 O 4 in the wave number of 400-4000 cm −1 .Magnetic hysteresis loop analysis of nano-Fe 3 O 4 and MACF@Fe 3 O 4 prepared with different mass ratios was performed using a Physical Property Measurement System (PPMS).The crystal forms of CACF and MACF@Fe 3 O 4 were analysed between 2θ = 20°−80° using the Ultima IV X-Ray Diffractometer of Rigaku Corporation, Japan.The pyrolysis mass loss of CACF and MACF@Fe 3 O 4 was analysed by STA449F3 synchronous thermal analyser of NETZSCH Company in Germany.

Optimization of adsorption conditions
The 50 ml of MB with different initial concentrations was taken, 0.010 g MACF@Fe 3 O 4 was added, the pH of the mixture was adjusted to a fixed value and the mixture was oscillated at 25°C for 60 min.The supernatant was collected and the concentration of the MB solution was monitored using a UVmini-1280 ultraviolet-visible spectrophotometer.The effects of the initial MB concentration, pH and adsorption time on adsorption performance were investigated.

Study on recycling performance
Under the optimized ideal working conditions, the MACF @ Fe 3 O 4 after one use was separated by suction filtration, dried in vacuum at 70°C and then reused for 5-6 times.The reaction time was 60 min and the adsorption rate of each use was determined.

Study on adsorption kinetics
Thirty-six samples of 0.010 g of MACF@Fe 3 O 4 were respectively added to 50 ml of 5.0 mg•l −1 MB and oscillated at a constant temperature of 25°C (150 r•min −1 ).The amount of MACF@Fe 3 O 4 adsorbed was measured with the change in adsorption time, and relevant kinetic data were obtained.The adsorption kinetics [19] of MB on MACF@Fe 3 O 4 were fitted by the pseudo-first-order, pseudo-second-order and intraparticle diffusion equations which were calculated using equations (2.3)-(2.5): is the adsorption capacity of MB at time t (min), Q e (mg•g −1 ) is the amount of adsorption at equilibrium and K 1 is a pseudo-first-order equation constant.
where Q t (mg•g −1 ) is the adsorption capacity of MB at time t (min), Q e (mg•g −1 ) is the amount of adsorption at equilibrium and K 2 is a pseudo-second-order equation constant. (2.5) where K id is a intraparticle diffusion equation constant, C (mg•g −1 ) is an empirical constant and Q t (mg•g −1 ) is the MB adsorption capacity at time t (min).

Study on adsorption isotherm
The adsorption isotherm describes the relationship between the adsorption capacity and the equilibrium concentration at a fixed temperature.Eighteen samples of 0.010 g MACF@Fe 3 O 4 were added into 50 ml MB solution (5.0, 15.0, 20.0, 30.0, 40.0 and 50.0 mg•l −1 ), respectively.The samples were shaken at 25, 35 and 45°C (150 r•min −1 ), and the absorbance value was measured after 150 min.The adsorption isotherms of MB on MACF@Fe 3 O 4 were calculated by the Langmuir model and Freundlich model.The corresponding equations are as follows: where C e (mg•l −1 ) is the concentration of MB after 150 min, Q e (mg•g −1 ) is the amount of adsorption at equilibrium, Q m (mg•g −1 ) is the maximum adsorption capacity, K L is constant of the Langmuir model, K F is constant of the Freundlich model and 1 n is the constant of adsorption strength.

Adsorption thermodynamics
Thermodynamic parameters can reflect the energy changes in the adsorption process, mainly including adsorption entropy variable (ΔS°), adsorption enthalpy variable (ΔH°) and Gibbs free energy variable (ΔG°), which are helpful to judge whether the adsorption reaction can occur on its own and its adsorption nature.According to the adsorption isotherm equilibrium data at different temperatures, the thermodynamic analysis was carried out by using Van 't Hoff equation and thermodynamic relationship: where K c is the partition coefficient, Q e (mg•g −1 ) is the amount of adsorption at equilibrium, C (mg•l −1 ) is the concentration of remaining solution, R (8.314 J•mol −1 •K −1 ) is the gas molar constant and T (K) is Fahrenheit degree.

Effect of mass ratio on adsorption and magnetic separation
The effect of MACF@Fe 3 O 4 which was prepared using mass ratios of CACF to nano-Fe 3 O 4 of 4:1, 2:1 and 1:1, on the adsorption rate and adsorption capacity of MB is shown in figure 1, and the effect on magnetic separation is shown in figure 2. Figure 1 shows that the MB adsorption rate of MACF@Fe 3 O 4 decreased with increasing nano-Fe 3 O 4 addition.When nano-Fe 3 O 4 was not loaded, the adsorption rate of MB by the CACF was 78.74%.At a mass ratio of 1:1, the adsorption rate of MACF@Fe 3 O 4 was 68.44%, which was 13.08% lower than that of the CACF.Simultaneously, the decrease in the rate of MB absorption by MACF@Fe 3 O 4 began to slow, indicating that the adsorption of MB by MACF@Fe 3 O 4 tended to be balanced, and the addition of excess nano-Fe 3 O 4 increased the preparation cost.Figure 2 shows that with an increase in nano-Fe 3 O 4 addition, the magnetic separation of MACF@Fe 3 O 4 was better.When the mass ratio was 1:1, most of the MACF@Fe 3 O 4 was attracted to one side by the magnet, and the magnetic separation was better than that of the sample with a mass ratio of 2:1.Wei [18] also adopted the method of electrostatic self-assembly to prepare Fe 3 O 4 @GO particles at a mass ratio of 1:1, which can be easily dispersed in solution and separated using ordinary magnets.Figure 3 shows the magnetic hysteresis loop of MACF@Fe 3 O 4 prepared at different mass ratios.The hysteresis loops of MACF@Fe 3 O 4 with different mass ratios are S-shaped, indicating superparamagnetic characteristics.However, with the decrease in nano-Fe 3 O 4 addition, the saturation magnetization (Ms) decreased and the magnetism gradually weakened.In summary, to enhance the magnetic separation ability of CACF and maximize the adsorption effect of MACF@Fe 3 O 4 , as well as possible, the mass ratio of CACF to nano-Fe 3 O 4 was 1:1.

Effect of preparation time on adsorption and magnetic separation
Figure 4 shows the effect of the preparation time of MACF@Fe 3 O 4 on the adsorption rate of MB.
It can be seen that the MB adsorption by MACF@Fe 3 O 4 decreased first and then increased.During 10-50 min, the adsorption rate of MB on MACF@Fe 3 O 4 gradually decreased with increasing preparation time.The sites on the surface of the CACF were gradually occupied by nano-Fe 3 O 4 , so that the adsorption rate decreased when adsorbing MB.However, MACF@Fe 3 O 4 still had a good adsorption effect, which was consistent with the results of Zhu-qing et al. [20].The MB adsorption rate of MACF@Fe 3 O 4 increased to 81.94% at 60 min.This may be because some of the nano-Fe 3 O 4 on MACF@Fe 3 O 4 was desorbed.Therefore, more sites on MACF@Fe 3 O 4 could adsorb MB and the adsorption rate increased.
Figure 5a,b shows the magnetic separation diagrams for the preparation times of 10 and 20 min, respectively.Comparing figure 5a,b , it can be seen that MACF@Fe 3 O 4 with a preparation time of 20 min could be cleanly separated by the magnet, while MACF@Fe 3 O 4 with a preparation time of 10 min had a poor magnetic separation effect.As shown in figure 5b, many MACF@Fe 3 O 4 powders remained floating in the sample bottle and were not attracted to the magnet.In summary, a preparation time of 20 min had a better magnetic separation effect and better retained the adsorption capacity.

Effect of pH on adsorption
MB is an alkaline dye.When ionized in an aqueous solution, the coloured ions are cations (MB + ).The surface of CACF may contain acidic oxygen (O)-containing groups such as carboxyl groups (-royalsocietypublishing.org/journal/rsos R. Soc.Open Sci.11: 240497 COOH), lactone groups (-COO-) and phenolic hydroxyl groups (-OH) [21].These acidic groups can be deprotonated under alkaline conditions and dissociated to form carboxyl anions (-COO -) and phenolic oxygen anions (-O -), which are beneficial for the electrostatic adsorption of MB + by MACF@Fe 3 O 4 .However, under acidic conditions, the dissociation of these acidic groups is inhibited.Han-bing et al. [21] found that the microporous structure of coconut shell AC modified with weak acids was better than that of coconut shell AC modified by strong acids.Therefore, the etching of weak acids on the carbon skeleton was beneficial for MACF@Fe 3 O 4 to improve the adsorption performance of MB.However, an acid or alkali concentration that was too high degraded the mechanical properties of MACF@Fe 3 O 4 .Part of the carbon skeleton of MACF@Fe 3 O 4 was oxidized, coupled with the etching effect on the carbon skeleton, resulting in a poor pore structure [22] and the adsorption rate was significantly reduced.
As shown in figure 6, the preparation pH had a significant effect on the MB adsorption rate of MACF@Fe    acidic or alkaline strength increased, the adsorption rate decreased significantly at pH 3.31 and pH 10.44 and the adsorption rate was the lowest at pH 3.31.Therefore, the optimal preparation pH for MACF@Fe 3 O 4 was 9.36.

Effect of temperature on adsorption
Figure 7 shows the effects of different preparation temperatures on the MB adsorption rate.An increase in the temperature inhibited the loading of nano-Fe 3 O 4 on the CACF.Under the preparation condition of room temperature (22.5°C), the adsorption rate was 89.62%.The lowest adsorption rate was 75.56%.At the preparation temperature of 50°C, the adsorption rate increased to 82.69% and continued to decrease with increasing temperature.Xian-hang et al. [23] found that the effect of temperature on the desorption effect was reflected in two aspects, the unfavourable factor is the density effect and the favourable one is the diffusion effect, which ultimately depends on the competition between the two.According to our previous research [17], most of the specific surface Ms (emu/g) 30 000 20 000 10 000 10 000 20 000 30 000  area and pore volume of CACF are provided by micropores (pore size <2 nm).Therefore, CACF are advantageous for the adsorption of small-molecule substances.However, the thermal motion of the water molecules in the solution increased and the density of nano-Fe 3 O 4 in the solution decreased when the preparation temperature increased.The density effect caused by the increase in temperature was greater than the diffusion effect, which was more conducive to the loading of nano-Fe 3 O 4 by the CACF.After loading, the adsorption sites on the surface of MACF@Fe 3 O 4 were occupied and the adsorption rate of MB decreased.When the preparation temperature was 50°C, the thermal motion of the water molecules was more intense and the diffusivity increased.At this time, the diffusion effect was greater than the density effect, and nano-Fe 3 O 4 was desorbed.The surface adsorption sites of the prepared MACF@Fe 3 O 4 were restored to the no-load state, thereby increasing the adsorption rate of MB.As the preparation temperature continued to increase, it was not conducive to the structural stability of MACF@Fe 3 O 4 , and the MB adsorption rate continued to decline.In summary, the ideal preparation temperature is 22.5°C.

analysis
From figure 8a,b, it can be seen that the surface of the CACF activated by KOH was clean and the pores were clear.Overall, the holes in the CACF were arranged and the cross-section presented an irregular honeycomb pore structure.The developed pore structure increased the surface area and enhanced adsorption, which was conducive to the loading of nano-Fe 3 O 4 .Figure 8a1,b1 show MACF@Fe 3 O 4 prepared at a mass ratio of 1:1 and preparation time of 20 min (pH 9.3, 25°C).The surface of MACF@Fe 3 O 4 became rough and the sizes of holes were significantly smaller.This structure endowed MACF@Fe 3 O 4 with magnetic separation characteristics while retaining its adsorption capacity for dye molecules to a large extent.The nano-Fe 3 O 4 was uniformly distributed in the form of particles, indicating that it was successfully attached to the surface of the CACF.

EDS analysis
The EDS and surface elemental composition analyses of CACF and MACF@Fe 3 O 4 are shown in figure 9a,b and table 1.As shown in table 1, the elements contained in the CACF were C and O, accounting for 83.72% and 15.23%, respectively.Possible reasons are as follows.First, there were some O-containing functional groups on the surface of the CACF after the KOH treatment.Second, the O in the air was adsorbed by the CACF.No iron was detected on the CACF, whereas MACF@Fe 3 O 4 contained a large amount of C and O, as well as a certain amount of iron.Iron was loaded onto the CACF in the form of oxide, indicating that nano-Fe 3 O 4 was successfully loaded onto the CACF.

FTIR analysis
Figure 10 shows that compared with CACF, the peak width of MACF@Fe 3 O 4 at the wave number of 3441 cm −1 was significantly enhanced.The broad peak is related to the O-H bond in the hydroxyl or carboxyl groups, indicating that the number of acidic O-containing functional groups increased after modification, which contributed to the adsorption of polar organic molecules.The absorption peaks of MACF@Fe 3 O 4 between 490 cm −1 and 690 cm −1 are regarded as the characteristic absorption peak of Fe-O, whereas there was no characteristic peak of Fe-O for the CACF.In the low-frequency region of Fe-O, the absorption peak at 490 cm −1 was attributed to the bending vibration of Fe-O-Fe, whereas the absorption peaks between 560 cm −1 and 690 cm −1 were attributed to the stretching vibration of

Magnetic hysteresis loops analysis
The magnetic properties of the samples were studied using a PPMS at room temperature (25°C) and an ±3 T external magnetic field.As shown in figure 11, the magnetic hysteresis loops of nano-Fe 3 O 4 and MACF@Fe 3 O 4 prepared under ideal conditions were S-shaped, and the thermoremanent magnetization and coercive force (Hc) were almost zero, indicating that both were spinel ferrite materials [26].In addition, the Ms of nano-Fe 3 O 4 was 64.5678 emu•g −1 , whereas that of MACF@Fe 3 O 4 was 48.2263 emu•g −1 .The Ms value of MACF@Fe 3 O 4 was smaller than that of nano-Fe 3 O 4 owing to the presence of non-magnetic CACF in MACF@Fe 3 O 4 .In summary, both nano-Fe 3 O 4 and MACF@Fe 3 O 4 have superparamagnetic characteristics, which makes the powdered MACF@Fe 3 O 4 easy to separate magnetically from the solution.This characteristic contributed to the effective dispersion of MACF@Fe 3 O 4 to adsorb MB from the aqueous solution and promoted the magnetic separation of MACF@Fe 3 O 4 from the treated solution using an external magnetic field.

XRD analysis
The XRD patterns of CACF before and after loading are shown in figure 12.As shown in figure 12, the pattern of MACF@Fe 3 O 4 was a broadened diffraction peak, indicating that MACF@Fe 3 O 4 existed in the form of amorphous C with low crystallinity composed of graphite crystallites.Compared with the CACF before and after loading, at 2θ = 29.40°and 43.19°, there were a graphite crystal (002) and ( 101) diffraction peaks [27], and both were wide peaks, indicating that the structure of CACF material was mainly in the form of graphite crystallite underdevelopment.Comparing the patterns of MACF@Fe 3 O 4 and nano-Fe 3 O 4 , it can be seen that the characteristic diffraction peaks of 57.24° (400) and 62.90° (422) appeared in the XRD pattern of MACF@Fe 3 O 4 , indicating that nano-Fe 3 O 4 was successfully loaded onto the surface of CACF in this experiment [28].As shown in figure 12, the coverage of MACF@Fe 3 O 4 was high.The peak at C element (002) became weak, indicating that the main form of iron loaded on CACF was nano-Fe 3 O 4 , and the presence of the load occupied the pores of the coconut shell AC, resulting in the weakening of the C crystal structure [29], thus proving the above point of view.

TG analysis thermogravimetric analysis
Figure 13 shows the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of CACF and MACF@Fe 3 O 4 from 25°C to 1100°C at a heating rate of 10 K•min −1 , respectively.Throughout the TG and DTG curves, it can be seen that the decomposition process of CACF and MACF@Fe 3 O 4 was relatively fast, and only one decomposition occurred during the heating process, so there was only one weight loss step.The peak value of the DTG curve appeared at about 50°C, corresponding to the temperature with the largest mass change rate, which was considered to be the water evaporation stage.Subsequently, the remaining mass was large, the decomposition temperature was high and the decomposition process was long.The difference is that the residual mass fraction of MACF@Fe 3 O 4 was higher after the test, and it was still not balanced at 1100°C, indicating that a large amount of nano-Fe 3 O 4 was not completely decomposed.

Effect of initial concentration on adsorption
Figure 14 shows the effect of the initial dye concentration on the adsorption performance.The adsorption capacity of MACF@Fe 3 O 4 for MB increased with increasing initial concentration [30], while the adsorption rate first increased and then decreased.At the initial concentration of 5.0 mg•l −1 , the maximum adsorption rate was 97.22%.When the MB adsorption capacity of MACF@Fe 3 O 4 at an initial concentration of 6.0 mg•l −1 was 14.22 mg•g −1 , it did not reach equilibrium or showed an equilibrium trend, that is, its adsorption capacity was not saturated.However, it can be seen from the adsorption rate curve that the adsorption rate of MACF@Fe 3 O 4 for MB began to decline.It can be speculated that if other conditions remained unchanged and the initial concentration continued to increase from 6.0 mg•l −1 , the adsorption of MB would reach saturation within a certain concentration range.

Effect of pH on adsorption
The molecular structure of MB (pK a = 4.52) contains MB + and the pH can affect the number of MB + ions in the solution [31].When 7.11 < pH < 9.97, the solution was in a weakly alkaline environment, and MB mainly existed in the form of MB + .The concentration of MB + increased with increasing pH.At this time, the surface of MACF@Fe 3 O 4 was negatively charged, resulting in electrostatic adsorption of MB + .At 7.11 < pH < 9.97, the adsorption capacity increased significantly with increasing pH.When the pH was 10.85, the adsorption capacity for MB decreased, which was considered to be caused by the destruction of the MACF@Fe 3 O 4 structure in a strong alkaline environment.In a weakly acidic environment, the number of MB + ions increased with increasing pH, but the surface of MACF@Fe 3 O 4 was positively charged.Therefore, the effect of electrostatic repulsion was not conducive to MB adsorption by MACF@Fe 3 O 4 .Second, H + and MB + in the solution competed for active sites, therefore, the adsorption capacity decreased with an increase in pH in the range of 5.09 < pH < 7.11.At pH <  4.52, the concentration of MB in the unionized state was higher than that in the ionized state; therefore, MB existed mainly in its molecular form.According to the s.e.m. results, MACF@Fe 3 O 4 had a rich mesoporous structure.Therefore, when 3.26 < pH < 4.09, a large number of dye molecules entered the pores of MACF@Fe 3 O 4 mainly through pore filling [32] rather than through electrostatic adsorption.
As shown in figure 15, adjusting the pH of MB to a weakly acidic or alkaline environment improved the adsorption capacity of MACF@Fe 3 O 4 .The maximum adsorption capacity of MACF@Fe 3 O 4 for MB was 11.26 mg•g −1 at pH 9.97.At a pH of 3.26, the lowest adsorption capacity was 10.07 mg•g −1 .In summary, the optimum pH for adsorption is 9.97.

Effect of time on adsorption
The adsorption capacity of MB with respect to adsorption time is shown in figure 16.The MB adsorption rate of MACF@Fe 3 O 4 increased with an increase in the adsorption time.The adsorption rate was very fast in the first 10 min, exceeding 50% after 7 min and reaching 90.53% after 45 min.At the beginning of the adsorption time, the concentration of MB was higher, and there were more active adsorption sites and O-containing functional groups on the MACF@Fe 3 O 4 .Therefore, the adsorption capacity gradually increased.In the later stage of adsorption, the growth rate of the adsorption capacity slowed down, and the active adsorption sites and O-containing functional groups on the surface of MACF@Fe 3 O 4 were gradually occupied.Equilibrium adsorption was reached at 110 min, and the adsorption rate was 97.29%, which showed that MACF@Fe 3 O 4 had a large adsorption capacity and a fast adsorption rate for MB.

Effect of time on adsorption
Figure 17 shows that MACF@Fe 3 O 4 shows good adsorption performance in the cycle experiment.After six cycles, the adsorption rate of MB by MACF@Fe 3 O 4 still remained above 90%, which indicated that MACF@Fe 3 O 4 had good stability and recycling performance, that is, the recovery rate of magnetic nano-Fe 3 O 4 was high.

Study on adsorption kinetics
The adsorption rate is closely related to the chemical mechanism.By analysing the adsorption kinetics, the adsorption behaviour of the adsorbent and dye can be determined when an adsorption relationship occurs [33].The adsorption kinetic model is often used to study the change rule of the adsorption rate of the adsorbent to the adsorbate under the influence of different factors.The above results for MACF@Fe 3 O 4 on the adsorption equilibrium of MB were used for the kinetic adsorption research.The processing time was fitted in three steps: 1-15, 20-50 and 60-120 min.The fitting curve of the pseudo-first-order equation when time (t•min −1 ) was plotted against ln(Q e − Q t ) is shown in figure 18.
The processing time (t•min −1 ) was plotted against t Q t , and the pseudo-second-order equation fitting curve is shown in figure 19.The fitting curve for the intraparticle diffusion equation is shown in figure 20, where t 1/2 is plotted against Q t .The relevant parameters for the kinetic model are shown in table 2.
At 1-15 min, the correlation coefficient R 2 of the pseudo-first-order kinetic was 0.9994, and that of the pseudo-second-order kinetic was 0.9686.The fitting degree of the pseudo-first-order kinetic model was higher, indicating that under the same conditions, the pseudo-first-order kinetic model could more accurately describe the dynamic adsorption process of MACF@Fe 3 O 4 on MB.The reaction rate of MACF@Fe 3 O 4 was mainly determined by the concentration of MB, which was similar to the pseudo-first-order kinetic reaction.The quasi-first-order kinetic equation was ln(Q e -Q t )=2.2394-0.0689t.At 20-120 min, the fitting degree of the pseudo-second-order kinetic was higher, indicating that the adsorption process was mainly controlled by chemical action during this period.The pseudo-secondorder kinetic equation was t Q t =0.0774+0.5295t.As shown in figure 20 and table 2 that the K id of Step 1 was 2.0587, which was greater than that of the other two steps, the correlation coefficient R 2 was the largest, and the fitting degree was the best, indicating that MB diffused more easily inside MACF@Fe 3 O 4 in the first 15 min.Over time, the active sites of MACF@Fe 3 O 4 were gradually occupied, and the adsorption process tended to be balanced.The K id of Step 3 was the smallest, indicating that this step was the control step of the entire adsorption process.In addition, the C values of the three steps in the intraparticle diffusion kinetic model were all greater than zero, indicating that the intraparticle diffusion kinetic equation did not pass through the origin and that the adsorption process rate was controlled by both intraparticle diffusion and boundary layer diffusion [34].

Study on adsorption isotherm
The adsorption kinetics of MB on MACF@Fe  temperatures were all less than 1.0, indicating that MACF@Fe 3 O 4 had a good adsorption effect on MB, and the affinity between the two was large.

Adsorption thermodynamic analysis
In order to reveal the thermodynamic characteristics of the adsorption of neutral red by ferromagnetic coconut palm ACF, the Van't Hoff equation and thermodynamic relationship were used to plot 1  T with lnK C and perform linear fitting.The corresponding ΔS°, ΔH° and ΔG° were calculated by the slope and intercept of the straight line, which are listed in table 4.
According to table 4, The ΔG° < 0 at different temperatures (25-45°C) indicates that the adsorption process of MB by MACF@Fe 3 O 4 was spontaneous.The value of ΔG° increased with the increase of temperature, indicating that the adsorption process at higher temperatures is credible.ΔH° > 0 indicating that the adsorption process was an endothermic process, and the temperature rise was conducive to the adsorption process.ΔS° > 0 indicates that the adsorption process of MACF@Fe 3 O 4 on MB increased the confusion of the solid-liquid interface.This may be because when MB was adsorbed on MACF@Fe 3 O 4 , water molecules were desorbed from it, and the entropy caused by water molecule desorption increased more than the entropy reduction caused by MB adsorption, so the overall was increased.

Discussion
This study is divided into three sections.Firstly, the ideal preparation and adsorption conditions of MACF@Fe 3 O 4 were determined.Secondly, MACF@Fe 3 O 4 was fully characterized, and the results confirmed the successful loading of nano-Fe 3 O 4 .Thirdly, the adsorption performance of MACF@Fe 3 O 4 was studied, including adsorption kinetics, adsorption isotherm and adsorption thermodynamics.The

Figure 1 .
Figure 1.Effect of mass ratio on adsorption rate.

Figure 4 .
Figure 4. Effect of preparation time on the adsorption rate.

Figure 6 .
Figure 6.Effect of preparation pH on the adsorption rate.

Figure 7 .
Figure 7. Effect of preparation temperature on the adsorption rate.

Figure 15 .
Figure 15.Effect of pH on the adsorption capacity.

Figure 16 .
Figure 16.Effect of time on concentration and adsorption rate.

Figure 17 .Figure 18 .
Figure 17.Effect of cycle number on adsorption rate of MB.

17
royalsocietypublishing.org/journal/rsos R. Soc.Open Sci.11: 240497 high-value utilization of coconut palm fibre waste is realized, and nano-Fe 3 O 4 endows it with excellent performance of convenient recycling.Therefore, MACF@Fe 3 O 4 is expected to be industrially applied urban sewage treatment, especially printing and dyeing wastewater treatment, and solve the problem of difficult recovery of powdered AC from a new perspective.

Table 2 .
Kinetic parameters of the adsorption of MB from MACF@Fe 3 O 4 .

Table 4 .
Thermodynamic parameters of adsorption of MB by MACF@Fe 3 O 4 .